This invention relates to a process for making tubes and other relatively thin-walled elongated shapes which facilitates attainment of a high degree of dimensional accuracy and stability and, more particularly, to such a process which is especially well suited for making such tubes and shapes which must have and retain a high degree of dimensional accuracy and stability after having been subjected to temperature cycling over an extremely broad range.
In the manufacture of elongated hollow shapes to precise, narrow tolerances, it has hitherto been known to utilize a technique frequently called "thermal sizing" in which the sizing force is the differential thermal expansion between two dissimilar materials. One of the materials is that of which the hollow elongated body is formed. The second material, which may be in the form of a mandrel to be enclosed by the body, is chosen so that its coefficient of thermal expansion is sufficiently greater than that of the body to be sized as to have the desired effect. The sizing force is developed when the body to be sized and the mandrel enclosed within it are heated and results from a change in the diameter or periphery of the mandrel with increasing temperature which stretches the body radially so as to increase its periphery.
Such a process is described by J. N. Suldan and R. J. Krahn* who used cast ductile iron having a coefficient of expansion of about 7.6 .times. 10.sup.- .sup.6 .degree.F.sup.-.sup.1 (13.68 .times. 10.sup.-.sup.6 .degree.C.sup.-.sup.1) and A.I.S.I. type 304 stainless steel with a coefficient of expansion of about 10.2 .times. 10.sup.-.sup.6 .degree.F.sup.-.sup.1 (18.36 .times. 10.sup.-.sup.6 .degree.C.sup.-.sup.1) to accomplish thermal sizing of tubes or cans made of Zircaloy-4 having a coefficient of expansion equal to about 3.6 .times. 10.sup.-.sup.6 .degree.F.sup.-.sup.1 (6.48 .times. 10.sup.-.sup.6 .degree.C.sup.-.sup.1); the stainless steel being used to accomplish the thermal sizing at a lower temperature than required for a mandrel formed of ductile iron. In that process, Zircaloy-4 strip, after being formed to the required thickness with a .+-. 0.004 inch (0.01 cm) tolerance, was shaped to semicylindrical shells having an internal radius equal to the external radius of the mandrel to be used for machining, welding and sizing the Zircaloy-4 tube. The strip was then mounted on the mandrel and TIG (tungsten inert gas) welded along the two longitudinal seams. Sizing was then carried out by annealing in vacuum at a temperature ranging from 900.degree. to 1450.degree.F (482.degree. to 788.degree.C), using the cast iron mandrel, and from 900.degree.F to 1170.degree.F (482.degree. to 632.degree.C), using the type 304 stainless steel mandrel. The thus formed tubes were then removed from the mandrels once the parts had cooled, and the tubes were checked, using contacting dial indicators, to determine dimensional accuracy. FNT *"Sizing of Zircaloy Structurals", February 1967, WAPD-TM-620, available from Clearinghouse for Federal Scientific and Technical information, National Bureau of Standards, U.S. Dept. of Commerce, Springfield, Va. 22151; abstract published Nuclear Science Abstracts Vol. 21, No. 11, June 15, 1967.
Suldan and Krahn indicate that
"a second sizing cycle, at the same temperature as the first and with the can rotated on the mandrel, may be required in order to achieve the required tolerances".* FNT *Ibid., p. 12.
Essentially the same process is described in U.S. Pat. No. 3,559,278 according to which a split sheet metal tubular body of the tube material is mounted on a mandrel which has a coefficient of linear expansion which is at least twice that of the tube material. The tube material is butt welded to form a tube on the mandrel with surface-to-surface contact between the thus formed tube and the mandrel. The tube and mandrel assembly are then heated to a temperature high enough for the mandrel to create sufficient tangential stress radially to expand the tube so that, upon cooling, the tube has the required lateral dimensions. The patent also points out that the straightness of the tube can be further improved by stretching it after the thermal sizing treatment.
Such processes have made possible the production of elongated hollow bodies to closer tolerance than had been previously possible; nevertheless, they left much to be desired. As was seen, one disadvantage resided in the need for repetitive thermal sizing in order to get the most out of the process. Further, such processes required that the tube or body-forming material be closely fitted to the mandrel, surface-to-surface contact being preferred in order to attain the desired thermal sizing. For many intended uses, even greater dimensional accuracy and stability, freedom from residual stresses and from distortion resulting from thermal cycling is desirable than was heretofore attainable.